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United States Patent |
5,534,999
|
Koshizuka
,   et al.
|
July 9, 1996
|
Monitoring sub-micron particles
Abstract
Sub-micron particles in fluid such as ultrapure water are detected or
monitored by a simple apparatus in which a light beam from a coherent
light source (1) is converged (2) in such a manner that the light beam is
focussed in a stream (3) of particle-containing fluid, the light passed
through the stream and diffracted by the particles is received by a
photo-detector (4) which is positioned at an opposite side of the coherent
light source with respect to the stream and substantially on an optical
axis of the light beam, so that the number of particles in the stream is
counted from electrical signals emitted by the photo-detector.
Inventors:
|
Koshizuka; Hiroshi (Shiga-ken, JP);
Kanatake; Takashi (Saitama-ken, JP)
|
Assignee:
|
Shinmikuni Kikai Ltd. (Osaka, JP)
|
Appl. No.:
|
383683 |
Filed:
|
February 1, 1995 |
Current U.S. Class: |
356/338; 356/336; 356/343 |
Intern'l Class: |
G01N 021/00; G01N 015/02 |
Field of Search: |
356/336,338,343
|
References Cited
U.S. Patent Documents
4178103 | Dec., 1979 | Wallace | 356/338.
|
4408880 | Oct., 1983 | Tsuji et al. | 356/338.
|
4522494 | Jun., 1985 | Bonner | 356/338.
|
4577964 | Mar., 1986 | Hansen, Jr. | 356/338.
|
4842406 | Jun., 1989 | Von Bargen | 356/338.
|
4850707 | Jul., 1989 | Bowen et al. | 356/338.
|
4917496 | Apr., 1990 | Sommer | 356/338.
|
5037202 | Aug., 1991 | Batchelder et al. | 356/338.
|
5085500 | Feb., 1992 | Blesener | 356/338.
|
5125737 | Jun., 1992 | Rodriguez et al. | 356/338.
|
5142140 | Aug., 1992 | Yamazaki et al. | 356/338.
|
Foreign Patent Documents |
62-803 | Jan., 1987 | JP.
| |
63-19535 | Jan., 1988 | JP.
| |
4-9635 | Feb., 1992 | JP.
| |
Primary Examiner: Hille; Rolf
Assistant Examiner: Ostrowski; David
Attorney, Agent or Firm: Kerkam, Stowell, Kondracki & Clarke, Feeney; William L.
Parent Case Text
This is a continuation of application Ser. No. 08/026,970, filed Mar. 5,
1993, now abandoned.
Claims
What is claimed is:
1. A method for measuring sub-micron particles in a fluid comprising
converging a light beam from a coherent light source (1) so that the
resulting focussed light passes through a stream (3) of fluid containing
particles therein in such a manner that a focus of the focussed light is
located in said stream (3) of fluid,
receiving a light passed through said stream (3) and diffracted by said
particles by means of a photo-detector (4) which is positioned at an
opposite side of said coherent light source (1) with respect to said
stream (3) and substantially on an optical axis of said light beam to
produce electrical signals, and
treating said electrical signals from said photo-detector (4) to count a
number of particles in said stream by using a predetermined calibration
curve; and wherein said focus is located in a focus portion of said stream
of fluid which has a cross sectional area at least as great as a maximum
cross sectional area in portions of said stream of fluid upstream and
downstream relative to said focus portion.
2. The method set forth in claim 1 wherein said coherent light source is a
laser diode.
3. The method set forth in claim 1 wherein said photo-detector comprises a
photo-diode alley.
4. The method set forth in claim 3 wherein said fluid is pure water or
ultrapure water.
5. The method set forth in claim 1 wherein said fluid is pure water or
ultrapure water.
6. The method of claim 1 wherein the light goes directly from the stream of
fluid to the photodetector.
7. The method of claim 6 wherein the focused light goes directly from a
focusing lens to the stream of fluid.
8. The method of claim 1 wherein the focused light goes directly from a
focusing lens to the stream of fluid.
9. An apparatus for measuring sub-micron particles in a fluid comprising
a coherent light source (1),
an optical system (2) for converging a light beam emitted out of said
coherent light source to produce a converged light,
a cell (3) through which a stream of fluid containing particles is flows
and being located in the neighborhood of a focus of said converged light
beam,
a photo-detector (4) which is positioned at opposite side of said coherent
light source (1) with respect to said stream and substantially on an
optical axis of said light beam to produce electrical signals, and
an electric circuit for treating said electrical signals from said
photo-detector (4) to count a number of particles in said stream by using
a predetermined calibration curve; and wherein said cell has a cross
sectional area at least as great as a maximum cross sectional area in
portions of said stream of fluid upstream and downstream relative to said
cell.
10. The apparatus set forth in claim 9 wherein said optical system consists
of a lens.
11. The apparatus set forth in claim 9 wherein said coherent light source
is a semiconductor laser.
12. The apparatus set forth in claim 9 wherein said photodetector comprises
a photo-diode alley arranged perpendicularly to the direction of said
stream and also perpendicularly to said optical axis.
13. The apparatus set forth in claim 12 wherein said electric circuit
includes differential amplifiers for multiplying signals from elements in
said photo-diode alley.
14. The apparatus of claim 9 wherein the light goes directly from the
stream of fluid to the photodetector.
15. The apparatus of claim 14 further comprising a focusing lens positioned
such that focused light goes directly from the focusing lens to the stream
of fluid.
16. The apparatus of claim 9 further comprising a focusing lens positioned
such that focused light goes directly from the focusing lens to the stream
of fluid.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method and apparatus for monitoring or
detecting sub-micron particles on a novel principle which is completely
different from the conventional techniques.
The method according to the present invention is advantageously applicable
to monitor and control impurity particles in fluids such as pure water and
ultrapure water used in electronics industries, biotechnology, medical and
pharmaceutical application and foods industries. The present method can be
used to evaluate the performance of separation membranes and filtration
systems.
2. Related Arts
The conventional methods for monitoring or detecting particles in fluid are
classified into following four categories:
(1) Shadow system in which decrement of light intensity caused by
travelling particles in fluid passing across an optical axis of parallel
ray.
(2) Microscope system in which fine particles in fluid are caught by a
membrane filter or the like and are observed or counted by a electron
scanning microscope.
(3) Light scattering system in which fluid is irradiated with an intensive
light such as a laser beam and the resulting scattered light is collected
by a lens so that the focused light is detected by an photo-multiplier
(4) Imaging system in which a fluid is irradiated with a light and the
resulting contrast of light is detected by a photo-diode alley and an
image of particles in the fluid is formed by a computer.
New techniques such as ultrasonic scattering technique are also proposed.
In the case of the shadow system (1), however, detection of fine particles
is limited to the particle size of about 1 .mu.m and hence this detection
system can not be used for sub-micron particles. In the microscope system
(2), more than half day is required to obtain the result.
The light scattering system (3) is the main current of development in
particle counters or detectors and now ultra-fine particles having the
particle size of less than 0.07 .mu.m can be detected by using a light
source having shorter wave length such as argon laser. In fact, Japanese
patent laid-open No. 4-39,635 discloses a technique to determine the
precise number of fine contaminant particles each having the particle size
of lower than 0.07 .mu.m contained in ultrapure water. This patent
proposes to use two detectors each receive the scattered light so that a
particle counter produces a signal when two detectors detect the scattered
light simultaneously. This system, however, requires a high-power laser as
well as very sensitive photo-multiplier, resulting in a large costly
system. Still more, in this system, precise alignment between an axis of
fluid stream containing particles to be detected and an optical axis is
required in order to assure the reliability of measurement. Japanese
patent laid-open No. 62-803 discloses an automated apparatus which
facilitates this alignment.
Japanese patent laid-open No. 63-19535 discloses a variation of the imaging
system (4). In this patent, a laser beam impinges vertically to a flow of
sample liquid and the diffracted and scattered light is passed through a
Fourier-transformation optical system or a lens to produce a Fraunhofer
diffraction image which is treated in order to evaluate fine particles in
the liquid. In this patent, a diameter of a laser beam is enlarged to
obtain a parallel ray which is directed to the sample liquid. This system
requires a complicated computer system.
Therefore, an object of the present invention is to provide a method which
permits to detect fine particles of sub-micron as contaminant in fluid, in
particular, pure water or ultrapure water by a simple and very economical
apparatus.
SUMMARY OF THE INVENTION
The present invention provides a method for detecting sub-micron particles
in fluid comprising converging a light beam from a coherent light source
so that the resulting focussed light passes through a stream of fluid
containing particles therein in such a manner that a focus of the focussed
light is located in the stream of fluid, receiving a light passed through
the stream of fluid and diffracted by the particles by a photodetector
which is positioned at an opposite side of the coherent light source with
respect to the stream and on an optical axis of the light beam to produce
electrical signals, and counting numbers of particles in the stream by
treating the electrical signals.
The present invention provides also an apparatus for detecting sub-micron
particles in fluid comprising a coherent light source, an optical system
for converging a light beam emitted out of the coherent light source to
produce a converged light, a cell through which a stream of fluid
containing particles flows and being located in the neighborhood of a
focus of the converged light beam, a photo-detector which is positioned at
an opposite side of the coherent light source with respect to the stream
and on an optical axis of the light beam to produce electrical signals,
and an electric circuit for counting numbers of particles in the stream by
treating the electrical signals.
The coherent light source is preferably a laser diode and the photodetector
comprises preferably at least one photo-diode, more preferably a
photo-diode alley arranged perpendicularly to the direction of the stream
and also perpendicularly to the optical axis. The optical system can be a
lens. Preferably, the electric circuit includes differential amplifiers
for multiplying signals from elements in the photo-detector alley. The
cell can be a part of a transparent tube through which a stream of fluid
containing particles flows.
The present invention is based on such surprising and unexpected finding
that the existence of sub-micron particles in a liquid stream can be
detected or monitored by utilizing diffraction phenomenon of a transmitted
light, which is observed when a converged coherence light is focussed on
the liquid stream. In fact, it is not known to use the transmitted light
of a converged light directly for detecting fine particles. In the
conventional detection technique, dispersed particles are irradiated with
an illumination parallel ray so that the resulting transmitted light image
is Fourier-transformed as is described in the Japanese patent laid-open
No. 63-19,535.
The detection principle of the method according to the present invention is
different from those of known methods but the theory why the particles in
fluid is detected precisely by the method according to the present
invention can not be explained completely at this stage. Following is one
of probable explanations.
Now, we will refer to attached drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a perspective view illustrating the principle of the detection
method according to the present invention.
FIG. 2 is an illustration for explaining a conventional technique.
FIG. 3 is an illustration similar to FIG. 2 but is for explaining the
detection method according to the present invention.
FIG. 4 is an example of a differential amplifier used in an apparatus
according to the present invention.
FIG. 5 and FIG. 6 are graphs each showing a relation between the particle
density in fluid and the number of particles detected in Example 1 and 2.
FIG. 7 and FIG. 8 are graphs each showing a relation between the number of
particles detected in ultrapure water after an ultrapurification unit
starts at different contamination levels
FIG. 9 shows graphs of development in contamination with bacteria in
ultrapure water left in ambient for four days.
At first, the conventional detection method is explained with referring to
FIG. 2 in which a fine particle (15) is irradiated with a parallel ray
produced by a laser (11) and collimator lens (12) and the resulting
diffracted light is converged by a lens (13). In this case, so-called
Fraunhofer diffraction image (20) shown at the right side of FIG. 2 is
observed. Fraunhofer diffraction is a well-known physical phenomenon which
is used in particle counters. A general term of "diffraction" is used to
define "all phenomena that can't be explained by the linearity of light"
and can be described by Fresnel-Huygens' principle. However, according to
this principle, the case when the radius of a particle becomes lower than
a wave length of light used can't be explained by the Fraunhofer
diffraction phenomenon but is described as scattering phenomenon of light.
In fact, if the radius of a particle becomes smaller than a wave length of
light used, the diffraction can't occur any more in a parallel ray because
each particle functions like a point source and scatters light
In much detailed scientific theory, the diffraction phenomenon is explained
as a kind of scattering phenomenon and can be described by the Mie
scattering theory which is derived strictly from the Maxwell's
electro-magnetic equation. Since the Mie scattering theory is complicated
and is difficult to be handled, an approximated equation is generally made
in the relation between the radius "r" of a particle and the wave length
".lambda." of light used (Rayleigh scattering for "r<.lambda.", Mie
scattering for the case when "r is nearly equal to .lambda.", Fraunhofer
scattering for "r>.lambda.").
According to the conventional Fraunhofer diffraction theory of a parallel
ray, the divergent angle .DELTA..theta. of diffraction caused by an
obstruction or fine particle is represented by an equation of
.DELTA..theta.=1.22 .lambda./D, in which "D" is a diameter of the particle
(D=2r). The divergent angle .DELTA..theta. of diffraction increases with
decrement of the diameter of the particle and becomes to 90.degree. when D
is equal to 0.78 .lambda.. Usually, this value of the divergent angle
.DELTA..theta. is the detection limit, so that the detectable minimum
particle size is 0.52 .mu.m at a wave length .lambda.=0.67 .mu.m. In fact,
a diffraction image of a particle whose particle size is lower than a wave
length used is not easily obtainable in experiments.
The present inventors found surprisingly such a fact that, when a particle
is placed in the neighborhood of a focus of a converged beam focussed by a
lens, the diffraction angle becomes so small so that such finer particles
becomes detectable, even if their particle sizes are smaller than the wave
length used. On this finding, the present inventors completed the present
invention which provides a novel method which permits to detect sub-micron
particles. The most important advantage of the method according to the
present invention in industry reside in that sub-micron particles can be
detected at high sensitivity and with high precision by a simple
combination of a cheap laser (light source) and a cheap photo-diode
(pickup).
In the method according to the present invention, the diffracted image can
be obtained for a particle whose particle size is smaller than 0.1 .mu.m
which is not observable in known techniques. Of course, particles having
the particle size of bigger than 0.1 .mu.m also can be detectable with
high sensitivity by the method according to the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Now, the present invention will be described with referring to drawings but
the present invention should not be limited to an embodiment shown in the
drawings.
An apparatus for monitoring or detecting fine particles shown in FIG. 1
illustrating principle of the detection method according to the present
invention comprises a laser (1) as a coherent light source, an optical
system, preferably a lens (2) for converging a light beam emitted out of
the coherent light source to produce a converged light, a cell (3) through
which a stream (5) of fluid containing particles flows, which is located
in the neighborhood of a focus of the converged light beam, a
photo-detector (4) which is positioned at an opposite side of the coherent
light source (1) with respect to the stream and substantially on an
optical axis of the light beam such as a photo-diode or a photo-diode
alley, and an electric circuit (not shown) for converting the resulting
light intensity signals or a diffraction image detected by the
photo-detector (4) to electrical signals from which numbers of particles
in the stream is counted. All elements used in the present invention are
available on market and are very cheap.
The coherent light source may consist of a laser (1) and a collimator lens
system (6) (FIG. 3). The laser (1) can be any laser but is preferably a
laser diode or semiconductor laser of small power. In other words, a cheap
laser diode can be used advantageously in the method according to the
present invention. Sensitivity increases with decrement of the wave length
of laser oscillation. Inventors confirmed that the detection principle of
the present invention can be applicable for a laser diode whose power is
smaller than 1 mW, for example 0.2 mW.
The focal distance of the optical system or lens (2) for converging a light
beam is determined in function of the particle size to be detected. For
example, a lens having the focal distance f=10 mm may be used to detecting
a fine particle whose particle size is 0.2 .mu.m.
The cell (3) must be transparent at least on light-receiving face and light
exiting face but can have a very simple structure because no consideration
is required to stray light. In other words, the light from the converging
lens 2 may go directly to the cell 3, meaning that no light shields or
lenses are disposed therebetween. The cell (3) has not necessarily a
rectangular section shown in FIG. 1 but can have any section. The cell (3)
can be a separate piece from a tube for a stream of fluid containing
particles but, according to another advantage of the present invention, is
preferably a part of a transparent tube through which the stream of fluid
containing particles flows. Thus, the portion of the fluid stream on which
the laser is focused is either larger in cross sectional area than the
tube (as shown) or the same diameter or cross sectional area as the tube
leading to the cell. The focus is located in a focus portion of the stream
of fluid which has a cross sectional area at least as great as a maximum
cross sectional area in portions of the stream of fluid upstream and
downstream relative to the focus portion. The transparent tube can be made
of fluoro resin in order to resist chemicals.
In practice, a suitable adjusting mechanism is preferably used for
positioning the optical system (2) so that the focused beam is focuses in
the neighborhood of the center of the cell (3).
Not so high sensitivity is required in the photo-detector (4) if the
photo-detector (4) can detect the diffraction image hidden in the
transmitted light. In this sense, photo-diode can be used. The
photo-detector (4) can comprise a single photo-diode but preferably
constitutes of a photo-diode alley. The photo-diode alley is preferably
arranged perpendicularly to the direction of the stream and also
perpendicularly to the optical axis. As shown in FIG. 1, the light goes
directly from the fluid stream to the photodetector 4, meaning that there
are no lenses therebetween.
The diffraction image or the distribution of intensity of a converged light
observed in the photo-detector (4) used in the apparatus according to the
present invention is illustrated at the right side of FIG. 3.
In practice, signals from elements in the photo-detector alley are
multiplied in differential amplifiers to improve the SN ratio of the
photo-detector (4) in such a manner that an electric signal of zero is
produced when the elements in the photo-detector alley are irradiated
uniformly or no particle passes through the cell (3), while suitable
electric signal which represents characteristics (number, size etc) of the
particles is produced when any change in intensity caused by the
diffraction image of a converged light is appeared in the elements in the
photo-detector alley.
FIG. 4 illustrates an example of differential amplifiers for a
photo-detector alley consisting of four photo-diode elements. The values
of resistances in the differential amplifiers of FIG. 4 are adjusted in
such a manner that zero output signal (e) is produced when identical
output signals are produced at the output (a to d) of all photo-diode
elements or the all photo-diode elements are irradiated with a light of
identical intensity. Therefore, the output signal (e) of the differential
amplifiers of FIG. 4 changes when any change in intensity is appeared in
the output signal, for example (a) of the photo-diode elements.
In the embodiments shown in FIG. 1 and FIG. 3, all elements of a laser (1),
a lens (2), a cell (3) and a photo-diodes (4) are arranged on a straight
line but they can be arranged on non-linear line by using suitable
mirror(s) in known manner so as to reduce the total size or length of the
apparatus.
Now, Examples of the monitoring/detection method according to the present
invention will be shown in Examples.
EXAMPLE 1
Fine particles were detected by using the principle shown in FIG. 1 under
following conditions and procedure:
______________________________________
Experiment conditions
______________________________________
Laser: Semiconductor laser
(wave length = 670 nm,
power = 0.5 mW)
Focal distance of a converging lens:
10 mm
Liquid tested: ultrapure water
Flow rate of the liquid:
100 mm/sec
Diameter of fine particles added:
0.208 .mu.m
Photo-detector: photo-diode alley
(32 elements)
Differential amplifiers:
FIG. 4
______________________________________
Experiment procedure
Three liquid samples having different particle concentrations (dilution of
1 to 3 times) were prepared and flows at a rate of 120 ml/min through the
apparatus illustrated in FIG. 1. The resulting change in the number of
particles detected is shown in FIG. 5.
This Example reveals such a fact that the method according to the present
invention is applicable to detect a particle having the particle size of
0.208 .mu.m.
EXAMPLE 2
Example 1 was repeated but the particle size of particles introduced in
ultrapure water was changed to 0.1 .mu.m.
The resulting change in the number of particles detected is shown in FIG.
6. This Example reveals that the method according to the present invention
is applicable to a system for detecting particles having the particle size
of 0.1 .mu.m.
EXAMPLE 3
The method according to the present invention was applied to ultrapure
water produced in an actual industrial water purification unit.
A curve "A" in FIG. 7 shows a relation between the number of particles
detected (arbitrary unit) by the method according to the present invention
and time duration after the purification unit starts.
For comparison, the same liquid sample was tested in two detectors of known
scattering method. Two curves "B" and "C" in FIG. 7 show the results
obtained by the known scattering systems in which makers of the detectors
indicate that particles above 0.1 .mu.m and 0.2 .mu.m are detectable
respectively.
EXAMPLE 4
Example 3 was repeated for another ultrapure water which is much purified
than Example 3.
The results of Example 3 and 4 reveal such a fact that much numbers of
particles are detectable in the method according to the present invention
comparing to the conventional scattering method.
From FIG. 5 to 8, it is apparent that the number of particles detected by
the method according to the present invention is substantially in
proportion to the number of particles actually present in liquid or to the
number of particles detected by the conventional scattering method.
Therefore, the method according to the present invention can be used as a
particle counter by using a suitable calibration curve.
EXAMPLE 5
The method according to the present invention was applied to ultrapure
water left in ambient atmosphere. Namely, the purity of ultrapure water
was examined daily for four days. The results are shown in FIG. 9.
In FIG. 9, a curve "C" show a relation between time duration (days) and the
number of colonies determined by a known culture technique in which
ultrapure water samples were cultured on medium.
Three curves of "A" and "B1, B2" are overlapped with the curve "C" in FIG.
9. The result of the method according to the present invention is shown by
the curve "A", while curves of "B1, B2" correspond to the conventional
scattering method.
FIG. 9 reveals such a fact that the result obtained by the method according
to the present invention has a stronger resemblance to the actual value
than the conventional scattering method. This means that the method
according to the present invention is suitable in medial uses or
pharmaceutical uses.
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